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1.1.3 Geographical coordinates These are derived from an ellipsoid and the origin of the coordinates is the centre of the ellipsoid.
h y usu lly f nc d o h ‘G nw ch’ meridian that runs through the Greenwich observatory just east of London in the UK. Meridians increase from 0° at Greenwich to 180° east and west of Greenwich. The ‘n n onl n ’ uns h ough h Pacific and is nominally at 180° east or west of Greenwich. However, different island groups in the Pacific decide to be one side or the other of the Date Line, and the line is drawn at various longitudes to defer to national boundaries. On older maps h ‘0’° m d n s no lw ys Greenwich. There are several other reference meridians, mainly in Europe. A list of these can be found in the EPSG parameter database. In order to facilitate loading of data in some software applications, the convention is that o h nd E s ‘pos ’ nd ouh nd West are negative. However the reader should beware that local applications that do not apply ou s d h ‘qu d n ’, m y not obey this convention.
Figure 4: Geographic coordinate system.
oj c on coo d n s f d o s ‘x’ nd ‘y’. However, h s c n b confus ng s n bou 50% of h wo ld, s ng s p s n d by ‘y’ nd no h ng by ‘x’. Caution is advised! usu lly c ll d s ngs nd no h ngs. om m s h y
More information about how these two types of coordinate system relate will be discussed in the following chapters.
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